The present invention relates to technologies for accurate detection of a pulse wave based on pulsations of an arterial vessel below an epidermal tissue of a living being, such as the carotid and thigh arteries, and for faithful indication of the detected pulse wave.
It is known in the art that blood pressure pulsations (hereinafter referred to as "pulse wave") of arteries caused by heartbeats of a living being represent working conditions of the heart, and consequently the detection of the pulse wave provides important medical information on troubles or diseases of the heart or the arterial vessels. For this reason, it has been practiced to diagnose blood circulatory organs of a subject by utilizing an apparatus which is capable of detecting and displaying such a pulse wave.
To obtain the pulse wave, such apparatus as indicated above is applied, for example and usually, to the carotid artery, which is a thick arterial vessel located right below the epidermal tissue and comparatively close to the heart. A commonly used apparatus includes a sensing unit which comprises a housing, a presser member, and an elastic member which is equipped with a strain gauge. The sensing unit is depressed onto the neck of a subject (e.g., a human being) such that the presser member protruding from the housing is pressed against the carotid artery via the epidermal tissue, and the pulsations (contracting and expanding movements) of the carotid artery which are transmitted as a carotid artery pulse wave to the elastic member through the presser member, are detected as an elastic deformation of the elastic member, which is converted by the strain gauge into electrical signals.
However, in the conventionally proposed apparatus of the type described above for detection of a carotid artery pulse wave, the elastic member pushing the presser member is directly secured and supported by to the housing structure, and therefore a pressure of depression between the carotid artery and the presser member (hereinafter called simply "depression pressure") is extremely easily and sensitively affected by a pressing force exerted to the housing of the sensing unit upon depression thereof onto the neck. Thus, the traditional apparatus suffers considerable difficulty in achieving faithful and accurate detection of the carotid artery pulse wave. Stated in more detail, the hypodermal tissue existing between the presser member and the carotid artery tends to function as an elastic body, whereby if the depression pressure is too low, the pulsating movements of the carotid artery, i.e., pulse waves are absorbed by the hypodermal tissue and accordingly the pulsations are not sufficiently transmitted to the presser member of the sensing unit, resulting in distortion of the pulse wave detected. On the other hand, if the depression pressure is excessively high, the pulsating movements of the carotid artery are impeded, with a result of failing to obtain the pulse wave of the carotid artery while it is normally functioning. Hence, to sufficiently faithfully detect the pulse wave of the carotid artery in its normally functioning condition in the form of vibrations of the presser member, it is necessary to depress the carotid artery with a pressure neither too high nor too low, i.e., with a suitable pressure, that is, to press the housing of the sensing unit against the neck of the subject with a suitable force. However, it is very difficult to make sure that the carotid artery is depressed with a suitable pressure, and also difficult to maintain the pressing force at a proper level during detection of the pulse wave. This difficulty has been a cause for the problem of distortion of the detected pulse wave.
Further, the faithful and accurate indication of a pulse wave of the carotid artery by the apparatus as described above, requires its protruding presser member to be depressed onto an outermost layer of the neck skin within a comparatively limited elongate area along a length of the carotid artery close to the surface of the skin layer. This means a need for a user of the apparatus to exercise an utmost care and a relatively high level of skill in positioning the sensing unit on the neck upon depression of the presser member, so that the pulse wave of the carotid artery can be obtained with high accuracy and reliability. In other words, a positional deviation of the presser member from the limited elongate area of the neck skin will result in a corresponding degree of absorption of the pulsations of the carotid artery by the hypodermal tissue of the neck, and lead to a resultant distortion of the pulse wave to be detected by the detecting apparatus.